package org.apache.lucene.search.highlight;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import java.util.HashSet;
import java.util.Iterator;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.Term;
import org.apache.lucene.search.BooleanClause;
import org.apache.lucene.search.BooleanQuery;
import org.apache.lucene.search.FilteredQuery;
import org.apache.lucene.search.Query;

/**
 * Utility class used to extract the terms used in a query, plus any weights.
 * This class will not find terms for MultiTermQuery, RangeQuery and PrefixQuery
 * classes so the caller must pass a rewritten query (see Query.rewrite) to
 * obtain a list of expanded terms.
 * 
 */
public final class QueryTermExtractor {

	/**
	 * Extracts all terms texts of a given Query into an array of WeightedTerms
	 * 
	 * @param query
	 *            Query to extract term texts from
	 * @return an array of the terms used in a query, plus their weights.
	 */
	public static final WeightedTerm[] getTerms(Query query) {
		return getTerms(query, false);
	}

	/**
	 * Extracts all terms texts of a given Query into an array of WeightedTerms
	 * 
	 * @param query
	 *            Query to extract term texts from
	 * @param reader
	 *            used to compute IDF which can be used to a) score selected
	 *            fragments better b) use graded highlights eg chaning intensity
	 *            of font color
	 * @param fieldName
	 *            the field on which Inverse Document Frequency (IDF)
	 *            calculations are based
	 * @return an array of the terms used in a query, plus their weights.
	 */
	public static final WeightedTerm[] getIdfWeightedTerms(Query query,
			IndexReader reader, String fieldName) {
		WeightedTerm[] terms = getTerms(query, false, fieldName);
		int totalNumDocs = reader.numDocs();
		for (int i = 0; i < terms.length; i++) {
			try {
				int docFreq = reader
						.docFreq(new Term(fieldName, terms[i].term));
				// IDF algorithm taken from DefaultSimilarity class
				float idf = (float) (Math.log((float) totalNumDocs
						/ (double) (docFreq + 1)) + 1.0);
				terms[i].weight *= idf;
			} catch (IOException e) {
				// ignore
			}
		}
		return terms;
	}

	/**
	 * Extracts all terms texts of a given Query into an array of WeightedTerms
	 * 
	 * @param query
	 *            Query to extract term texts from
	 * @param prohibited
	 *            <code>true</code> to extract "prohibited" terms, too
	 * @param fieldName
	 *            The fieldName used to filter query terms
	 * @return an array of the terms used in a query, plus their weights.
	 */
	public static final WeightedTerm[] getTerms(Query query,
			boolean prohibited, String fieldName) {
		HashSet terms = new HashSet();
		if (fieldName != null) {
			fieldName = fieldName.intern();
		}
		getTerms(query, terms, prohibited, fieldName);
		return (WeightedTerm[]) terms.toArray(new WeightedTerm[0]);
	}

	/**
	 * Extracts all terms texts of a given Query into an array of WeightedTerms
	 * 
	 * @param query
	 *            Query to extract term texts from
	 * @param prohibited
	 *            <code>true</code> to extract "prohibited" terms, too
	 * @return an array of the terms used in a query, plus their weights.
	 */
	public static final WeightedTerm[] getTerms(Query query, boolean prohibited) {
		return getTerms(query, prohibited, null);
	}

	// fieldname MUST be interned prior to this call
	private static final void getTerms(Query query, HashSet terms,
			boolean prohibited, String fieldName) {
		try {
			if (query instanceof BooleanQuery)
				getTermsFromBooleanQuery((BooleanQuery) query, terms,
						prohibited, fieldName);
			else if (query instanceof FilteredQuery)
				getTermsFromFilteredQuery((FilteredQuery) query, terms,
						prohibited, fieldName);
			else {
				HashSet nonWeightedTerms = new HashSet();
				query.extractTerms(nonWeightedTerms);
				for (Iterator iter = nonWeightedTerms.iterator(); iter
						.hasNext();) {
					Term term = (Term) iter.next();
					if ((fieldName == null) || (term.field() == fieldName)) {
						terms.add(new WeightedTerm(query.getBoost(), term
								.text()));
					}
				}
			}
		} catch (UnsupportedOperationException ignore) {
			// this is non-fatal for our purposes
		}
	}

	/**
	 * extractTerms is currently the only query-independent means of
	 * introspecting queries but it only reveals a list of terms for that query -
	 * not the boosts each individual term in that query may or may not have.
	 * "Container" queries such as BooleanQuery should be unwrapped to get at
	 * the boost info held in each child element. Some discussion around this
	 * topic here:
	 * http://www.gossamer-threads.com/lists/lucene/java-dev/34208?search_string=introspection;#34208
	 * Unfortunately there seemed to be limited interest in requiring all Query
	 * objects to implement something common which would allow access to child
	 * queries so what follows here are query-specific implementations for
	 * accessing embedded query elements.
	 */
	private static final void getTermsFromBooleanQuery(BooleanQuery query,
			HashSet terms, boolean prohibited, String fieldName) {
		BooleanClause[] queryClauses = query.getClauses();
		for (int i = 0; i < queryClauses.length; i++) {
			if (prohibited
					|| queryClauses[i].getOccur() != BooleanClause.Occur.MUST_NOT)
				getTerms(queryClauses[i].getQuery(), terms, prohibited,
						fieldName);
		}
	}

	private static void getTermsFromFilteredQuery(FilteredQuery query,
			HashSet terms, boolean prohibited, String fieldName) {
		getTerms(query.getQuery(), terms, prohibited, fieldName);
	}

}
